Abstract
Abstract. Load calculations play a key role in determining the design loads of different wind turbine components. To obtain the aerodynamic loads for these calculations, the industry relies heavily on the Blade Element Momentum (BEM) theory. BEM methods use several engineering correction models to capture the aerodynamic phenomena present in Design Load Cases (DLCs) with turbulent wind. Because of this, BEM methods can overestimate aerodynamic loads under challenging conditions when compared to higher-order aerodynamic methods – such as the Lifting-Line Free Vortex Wake (LLFVW) method – leading to unnecessarily high design loads and component costs. In this paper, we give a quantitative answer to the question of load overestimation of a particular BEM implementation by comparing the results of aeroelastic load calculations done with the BEM-based OpenFAST code and the QBlade code, which uses a particular implementation of the LLFVW method. We compare extreme and fatigue load predictions from both codes using sixty-six 10 min load simulations of the Danish Technical University (DTU) 10 MW Reference Wind Turbine according to the IEC 61400-1 power production DLC group. Results from both codes show differences in fatigue and extreme load estimations for the considered sensors of the turbine. LLFVW simulations predict 9 % lower lifetime damage equivalent loads (DELs) for the out-of-plane blade root and the tower base fore–aft bending moments compared to BEM simulations. The results also show that lifetime DELs for the yaw-bearing tilt and yaw moments are 3 % and 4 % lower when calculated with the LLFVW code. An ultimate state analysis shows that extreme loads of the blade root out-of-plane bending moment predicted by the LLFVW simulations are 3 % lower than the moments predicted by BEM simulations. For the maximum tower base fore–aft bending moment, the LLFVW simulations predict an increase of 2 %. Further analysis reveals that there are two main contributors to these load differences. The first is the different way both codes treat the effect of the nonuniform wind field on the local blade aerodynamics. The second is the higher average aerodynamic torque in the LLFVW simulations. It influences the transition between operating modes of the controller and changes the aeroelastic behavior of the turbine, thus affecting the loads.
Highlights
Load calculations are an essential process when designing large modern wind turbines
In this paper we analyzed the effect of two different aerodynamic models on the performance and especially on the loads of the Danish Technical University (DTU) 10 MW Reference Wind Turbine (RWT)
The first aerodynamic model – used in the aeroelastic simulation software OpenFAST – is an implementation of the Blade Element Momentum (BEM) method, the standard method used in the industry
Summary
Load calculations are an essential process when designing large modern wind turbines. International guidelines and standards prescribe a large number of aeroelastic simulations of the complete turbine for each load calculation loop (IEC 61400-1 Ed. 3, 2005). These simulations, or Design Load Cases (DLCs), are required in order to cover many possible situations that the wind turbine might encounter in its lifetime and calculate realistic loads. As the wind turbines become larger, the design loads of each component scale following a power law of the rotor diameter (Jamieson, 2018, 97–123). This leads to increased material requirements and to higher component costs. If overly conservative load estimates on these large multi-megawatt scales can be avoided, it would result in a considerable reduction in material use and component costs
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